Unlike conventional random access memory (RAM) chip technologies, in magnetic RAM (MRAM) data is not stored as electric charge but is instead stored by magnetic polarization of storage elements. The storage elements are formed from two ferromagnetic layers separated by a tunneling barrier layer. One of the two layers, called the fixed layer, has at least one reference magnetic polarization set to a particular polarity. The magnetic polarity of the other magnetic layer, called the free layer, is altered to represent either a “1” (e.g., anti-parallel to the fixed reference layer) or “0” (e.g., parallel to the fixed reference layer). One such device having a fixed layer, a tunneling barrier layer, and a free layer is a magnetic tunnel junction (MTJ). The electrical resistance of an MTJ is dependent on the magnetic polarity of the free layer compared to the magnetic polarity of the fixed layer. A memory device such as MRAM is built from an array of individually addressable MTJs.
To write data in a conventional MRAM, a write current is applied through an MTJ. The write current has a magnitude exceeding a level called the critical switching current level which is sufficient to change the orientation of spin or magnetization in the free layer. When the write current flows in a first direction, the MTJ can be placed into or remain in a first state, where its magnetizations are in a parallel orientation. When the write current flows in a second direction, opposite to the first direction, the MTJ can be placed into or remain in a second state, where its magnetizations are in an anti-parallel orientation.
To read data in a conventional MRAM, a read current may flow through the MTJ via the same current path used to write data in the MTJ. If the magnetizations of the MTJ are in a parallel orientation, the MTJ presents a resistance that is different than the resistance the MTJ would present if the magnetizations of the MTJ element were in an anti-parallel orientation. Thus, in a conventional MRAM, there are two distinct states defined by two different resistances, and logic “0” or a logic “1” value can be read based on the state.
Bitcells of a magnetic random access memory may be arranged in one or more arrays including a pattern of memory elements (e.g., MTJs in case of MRAM). STT-MRAM (Spin-Transfer-Torque Magnetic Random Access Memory) is an emerging nonvolatile memory that has advantages of non-volatility, comparable speed to eDRAM (Embedded Dynamic Random Access Memory), smaller chip size compared to eSRAM (Embedded Static Random Access Memory), unlimited read/write endurance, and low array leakage current.
In one category of MRAM memory cells, the direction of polarization of the free layer and the reference layer of a magnetic tunnel junction (MTJ) is parallel to the plane of the respective layer. Such memory cells are referred to as having in-plane magnetic anisotropy or longitudinal magnetic anisotropy (LMA). In another category of MRAM memory cells, the direction of polarization of the free layer and reference layer of an MTJ is perpendicular to the plane of the respective layer. Such memory cells are referred to as having perpendicular magnetic anisotropy (PMA).
The critical switching current, which is the amount of electrical current that can cause an MRAM memory cell to change from one logical state to another, is also referred to as the switching current of the memory cell. PMA type memory cells use less switching current than LMA type memory cells. It would be desirable to improve the perpendicular magnetic anisotropy of MRAM memory cells to produce MRAM devices with reduced power consumption.